The present invention relates to pilot operated proportional hydraulic valves, and particularly to such valves which are electrically controlled.
Industrial equipment, such as lift trucks, have moveable members which are operated by hydraulic cylinder and piston arrangements. Application of hydraulic fluid to the cylinder traditionally has been controlled by a manual valve, such as the one described in U.S. Pat. No. 5,579,642. A manual operator lever was mechanically connected to move a spool within the valve. Movement of the spool into various positions with respect to cavities in the valve body enables pressurized hydraulic fluid to flow from a pump to one of the cylinder chambers and be drained from another chamber. The rate of flow into the associated chamber is varied by varying the degree to which the spool is moved, thereby moving the piston at proportionally different speeds.
Because the manual valves are mounted in the operator cab of the equipment, individual hydraulic lines have to be run from the valve to the associated cylinders. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located in the operator cab. Instead, the solenoid valves are mounted adjacent the associated cylinders, thereby requiring that only a hydraulic line from pump and another line back to the fluid tank need to be run through the equipment. Although electrical signals have to be transmitted from the operator cab to the solenoid valves, wires are easier to run and less prone to failure than pressurized hydraulic lines.
Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. A spring acts on the armature to close the valve when electric current is removed from the solenoid coil. One type of solenoid operated pilot valve is described in U.S. Pat. No. 5,878,647.
Industrial lift trucks require that the boom be capable of being lowered in a controlled manner in the event of a failure of the hydraulic or electrical systems. This is easily accomplished when the hydraulic actuators are controlled by valves in the operator cab. However, remotely located solenoid valves make the manual lowering difficult to achieve.
The present invention provides a mechanism for operating a hydraulic actuator in a controlled manual manner in the event of a failure of either or both of the hydraulic or electrical systems. This mechanism is particularly adapted to equipment with solenoid valves in remote locations adjacent to the hydraulic actuators.
A pilot operated control valve has a main valve poppet that selectively engages a valve seat to control flow of fluid through the valve. As in conventional pilot valves, pressure in a control chamber on a side of the main valve poppet that is remote from the valve seat, determines the position of the poppet and thus the open or closed state of the valve. During normal operation the pressure in the control chamber is determined by a pilot poppet that is operated by a device, such as an electromagnetic solenoid.
The present invention adds a discharge valve that can be employed during an emergency to open the pilot operated control valve. The discharge valve responds to a control signal by releasing pressure in a control chamber of pilot operated control valve and move the main valve poppet to an open position.
In the preferred embodiment of the emergency override mechanism, an auxiliary bore is formed in the body of the pilot operated control valve. A discharge poppet is slidably located within the auxiliary bore, thereby defining a first cavity on one side of the discharge poppet and a second cavity on another side of that poppet. The first cavity receives the control signal and the second cavity is coupled to a fluid reservoir. The discharge poppet also forms an intermediate cavity in the auxiliary bore, which is coupled to the control chamber of the pilot operated control valve. The position of the discharge poppet in the pilot operated control valve regulates flow of fluid between the control chamber and the second cavity in response to the control signal pressure in the first cavity.
A manually operated hand pump or other source of pressurized fluid is used in an emergency to apply the control signal to the discharge poppet and open the pilot operated control valve.